BUG#25659 - memory leak via "plugins" test

Re-enabled plugin test to check if it still leaks memory.


mysql-test/t/disabled.def:
  Re-enabled plugin test.

mysql-test/t/disabled.def

@@ -35,7 +35,6 @@ synchronization          : Bug#24529  	Test 'synchronization' fails on Mac pushb
 #ndb_binlog_discover      : bug#21806 2006-08-24
 #ndb_autodiscover3        : bug#21806
 
-plugin                   : Bug#25659 memory leak via "plugins" test
 #rpl_ndb_dd_advance	 : Bug#25913 rpl_ndb_dd_advance fails randomly
 
 rpl_ndb_stm_innodb       : Bug#26783